IgA nephropathy (IgAN) is the most common form of glomerulonephritis, an inflammatory disease of the kidney. IgAN is characterized by deposition of lgA1 antibodies in the glomerular mesangium of the kidney, followed by infiltration of inflammatory immune cells and eventual loss of kidney function. lgA1 antibodies show altered glycosylation and polymerization states in a large percentage of IgAN patients. These alterations have been suggested to play a role in modifying the interactions between lgA1 and IgA-specific receptors, particularly, FcalphaRI (CD89) and transferrin receptor (TfR), although conflicting data have been reported. The long-term goal of this research effort is to ascertain the precise molecular events that lead to lgA1 deposition in the mesangium. The central hypothesis is that modifications in lgA1 glycosylation and polymerization lead to increased affinity for critical IgA-binding receptors (specifically, FcalphaRI and TfR). These aberrant interactions are proposed to lead directly to deposition of lgA1 in the kidney mesangium and the ensuing inflammatory response. Specific Aims for this proposal include: 1) Characterize the effects of lgA1 glycosylation and polymerization on its interaction with FcalphaRI and TfR, using recombinant lgA1 with defined glycosylation states;2) Characterize the receptor interactions of lgA1 purified from serum of healthy and IgAN patients and correlate the receptor affinity with lgA1 glycosylation and polymerization state;and 3) Determine the contribution of FcalphaRI N-glycosylation to its interaction with lgA1 and TfR. Binding studies with lgA1, FcalphaRI, and TfR will be carried out by surface plasmon resonance and analytical ultracentrifugation. lgA1 glycosylation will be characterized by lectin affinity assays along with thorough analysis of samples of interest by Fourier transform-ion cyclotron resonance mass spectrometry. This research will help determine the molecular basis for IgA nephropathy (IgAN), a kidney disease affecting approximately 100,000 patients in the US. IgAN is a primary cause of end-stage renal disease, which costs $18 billion per year in medical care. Understanding the molecular interactions underlying IgAN will be the first step towards developing treatments for this costly and debilitating disease.